Frequency discriminating network



`June 4,1940.. v w. R. nourri 2,203,49

FHEQUENY DISUBIMINNI'ING NETWORK Filed Sept 3U, 19:57

s sheets--sheet l 3 :mentor June 4, 1940. -W, R, KOCH 2,203,498

FREQUENCY DISCRIMINATING NETWORK Filed Sept. 30, 1937 5 Sheets-Sheet 2June 4, l.

w R. KOCH 2,203,498

FREQUENCY DISCRIMINATING NETWORK Filed sept. 5o, 19:57

:s sheets-sheet 5 :inventor Cttorneg Patented June 4, 1940 UNITED STATESFREQUENCY DISCRIMINATING NETWORK Winfield R. Koch, Haddonfield, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication September 30, 1937, Serial No. 166,495

10 Claims.

This invention relates to frequency discriminating networks for radiosignaling systems including automatic frequency and volume controllingmeans, and has for its `principal object the provision of an improvedapparatus and method of operation whereby automatic control of thefrequency and signal volume in such systems may be effected without the`expense and disadvantages heretofore incident to the construction andoperation of such devices. i

Automatic frequency control circuitsheretofore available have usuallyincluded a center tapped winding, separate primary and secondarywindings, a blocking condenser and choke coil, a separate detectorcircuit, a tube capable of performing a plurality offunctions orcombinations of certain of these features.` Such circuits are expensiveboth because of the accurate center tapping required and because of thechoke coil involved and are not altogether satisfactory for the reasonthat the diode used for securing audio frequency potentials hasimpressed on it the signal potentials from both the primary `and thesecondary of the coupling transformer. Under these circumstances,'notonly is the full selectivity of the transformer not utilized but thevoltage impressed on the diode varies throughout the desired frequencyband, or channel, instead of remaining substantially constant. i

` In accordance with the present invention', these difficulties areminimized or avoided by the use of means including a pair of diode tubeelements so interconnected as (l) to detect the modulation or audiocomponent of the signal, (2) to provide suitable automatic volumecontrol potentials, and (3) to provide suitable automatic frequencycontrol potentials. More specifically stated, the modulated signal orintermediate frequency impulses are supplied through a couplingtransformer to one diode which detects the modulation component andaffords one component of a resultant unidirectional voltage whichis'utilized in effecting automatic control of the amplitude `ofthecarrier frequency. lAnother diode is subjected to the resultantof thecoupling transformer primary and secondary voltages for producinganother unidirectional potential which varies with change in the phaserelation between the primary and secondary voltages, and whichiscombined with the substantially constant unidirectional component ofthe detected potential for automatically regulating the carrierfrequency. For the automatic volume control potential derived from theoutput of the signal detector, suitable filters are provided to exclude(Cl. Z-20) Figure 1 is a wiring diagram of a preferred embodiment of theinvention wherein the automatic frequency control potential is derivedfrom series- I connected resistors,

Figures 2 to 4 are `explanatory diagrams relating to the operation ofthe invention,

Figure 5 illustrates thev application of the invention to a tuned radioreceiver including groundedrotor condensers, the automatic frequencypotential being in this case derived from a bridge network instead ofseries-connected resistors,

Figure 6 illustrates a modification differing from those of Figs. 1 and5 in that substantially constant component of the automatic frequencycontrol potential is derived from the transformer primary instead of`the transformer secondary, thus permitting the following tube tofunction either as a detector or amplifier,

Figure 7 illustrates a modication which differs from the previousmodifications in that the automatic frequency control potential derivedfrom the network has the opposite polarity at any frequency from thatderived from the network in Fig. 1. This is desirable for some types offrequency control means,

Figure 8 exemplifies a modification in which only part of the primaryvoltage is utilized, and

Figure 9 illustrates a modification including a voltage doubler detectorarranged to produce the automatic frequency control potential through abridge network.

The circuitof Fig. 1 includes an input transformer I0, an intermediate`frequency amplifier tube II, a coupling transformer I2, a detector I3, a`coupling resistor I4 and an audio frequency amplifier tube I5. Ashereinafter referred to, the relative frequency response between theprimary and secondary voltages within the pass band of the transformerI2, which is of the tuned type, is shown in Figure '2. Connected betweenthe primary winding IB and the secondary winding I1 of the couplingtransformer I2 is a circuit including capacitors I8 and I9 and a diode20 which is shunted by a pair of resistors 2| and 22. Automaticfrequency control potential dependent on the unidirectional potentialdrops of the resistors I4 and 22 is applied to the lead 23, impulses ofaudio frequency being excluded from this lead by lter means such as thecapacitor I9 and a resistor 24 interposed between the resistors I4 and22. Impulses of both radio and audio frequencies are further excludedfrom this lead by the capacitor-resistor filter 25-26. The capacitors I8and I9 afford direct current isolation for the diode 2U, thus permittingthe series connection of the resistors I 4 and 22.

Automatic volume control potential is applied to a lead 21 from theoutput ofthe detector I3 through a resistor 28 which functions with acapacity 29 to form a filter whereby audio and radio frequency impulsesare excluded from this automatic Volume control lead 21. The resistor 24also prevents radio frequency impulses froml getting into the automaticvolume control circuit.

The operation of the circuit of Fig. 1 will be readily understood.yW'here exact tuning is in the center of the `frequency band, theprimary and secondary voltages of the transformer I 2 are displaced inphase by approximately 90 degrees for the reason that they provide twocoupled circuits, tuned to the same frequency. The resistors 2I and 22on which the resultant of the primary and secondary voltages isimpressed are so proportioned that, at exact tuning, the potential dropof resistor 22 is equal to and opposed to that of resistor I4. Underthese conditions, the Aautomatic frequency control potential is zero atexact tuning. SinceY the unidirectional potential drop of the resistorI4, that is, the rectified secondary voltage, is substantiallyindependent of change in frequency in the region of exact tuning due tothe band-pass characteristic of the transformer I2 that is, band passtuning response in a relatively narrow band about the mean frequency of460 kc. as indicated by the response curves of Figure 2, and the drop ofresistor 22 changes rapidly with change in frequency, there is appliedto the autor matic frequency control lead 23 a potential which is toolow and negative when the frequency is too high or vice versa, accordingto the frequency adjusting element to be controlled, and depending onthe direction of the coupling between the primary I and the secondary I1of the coupling transformer.

The rectified primary or secondary voltage is balanced against thevector sum of the primary and secondary voltages rectified thereby toproduce a composite output potential. At resonance the drop in resistors22 and I4 are equal and the automatic frequency control or compositeoutput voltage is Zero. When the primary and secondary voltages are inphase the voltage in resistor 22 is zero and the voltage in resistor I4is positive as applied to the output or AFC circuit 23. When the primaryand secondary voltages are fully out of phasethe potential drop inresistor 22'is equal to twice the drop in the resistor I4 and theautomatic frequency or composite output voltage in resistor I4 isnega-tive as applied to the output lead 23.

As indicated by the curve of Fig. 4, this automatic frequency controlvoltage is substantially linear in the region .of exact tuning which inthis particular case is assumed to be in the neighborhood of 460kilocycles. This control potential ltroltuning capacitor 3U.

, sumed to occur at 460 kilocycles.

Fig. 5 illustratesq the application of the invention to atune'd radiofrequency receiver including a grounded-rotor;,multiple-section uni-con-Where these circuits are tuned tothe incoming signal frequency, a menchanical frequency control device or an electronic reactance across eachtuned circuit would ordinarily'v'beyrequired. Where the adjustablefrequency circuits form the intermediate frequency amplifier', theAinvention may be used to regulate the frequency -of`thelocal oscillator.Tov this end, the substantially frequency independent andfrequency'responsive control voltages are produced inthe resistors I4and 22 as previously indicated and the resultant of these voltages isapplied to the automatic frequency control lead 23 'through resistors32'and 33 which are interconnected with the resistors I4 and 22 to forma bridge network which will be balanced at exact tuning,"to give vzerofrequency controlv potential. By proper choice of circuit constants, theresistor inthe l'ead"'23 of Fig) l'may be omitted as shown in Fig. 5IIn1thiscase, filteringz is effected by theirss'to'rs 32 and' 33." 1

The" arrangement o'fFig. Gdiffers from the previous modifications inthat' a separate detectoror amplifier 34 is supplied from the secondaryof the transformer l`2'and a separate diode 35 in place of the diode I3-in Figure 1, coupled through a capacitor; iSd to thetransformer primaryI 6 instead of to the secondary, isutilized to produce in'a resistorytti the automatic volume'control potential andthe y"frequencyindependent com-- ponent of the automatic frequency control potential.This connection hasthe advantage' that somewhat 'better' automaticvolume control'is secured' sincey the primary is more broadly tuned thanthe secondary. Fig. 6 'also illustrates an arrangement wl'iicl'i differsfrom that of Fig. 1 in thatl the frequency'independent component of theautoma-tic frequency control potential is derived from the,automaticvolume control lead 21, instead of from? the anode end of the filterresistor 28. This assures thaty'no'audio frequency voltage willbeappliedy to the diode ZIJ. This potential is then balancedwith thevector sum of the rectified primary and secondary voltages byconneyction through a resistor 24 to the diode 20 outputrresistor 22,las in Figure l. l In the 'circuit ofFig. '7, the automatic volumecontrol potential applied to `the lead 21 is amplilfied byv an`'amplifier '31 'interposed between the double diode' andthe controlgrids of the controlledtubes suchas the tube iI.` This requires that theamplifier grid controlr potential derived from the resistor I4` bepositively polarized for the reason that the/tube 3l effects a reversalof polarity and the control grids of the controlled tubes such as thetube II, lmay be subjected to a negative bias potential.v 4ln this casethe automaticfrequency control potential derived from the .resistorsMand22 goes from plus tominus as the'frequencyincreases. This wouldpermit the use of an amplifier similar to 31, for amplifying 40 ence isthe derivation of the automatic frequency the automatic frequencycontrol potential, if desired, or permit the use of a frequency control`means requiring opposite control voltages.

Fig. 8 illustrates an arrangement that is similar to that of Fig. l, butdiffers therefrom in that only a portion of the primary voltageisimpressed on the cathode of diode 2|] by reason of the tapped capacitorconnection 3I-3I. Where very loose coupling is used between primary I 5and secondary II of the coupling transformer I2,

l the primary voltage may be several times as large as that 'of thesecondary. Also, under these conditions, the capacitive reactance of thediode 20 age divider 3 I -3I shown is in the capacitive pori tion of theprimary tunedcircuit, it is readily apparentthat voltage division maybeaccomplished in the inductive portion ofthe tuned primarycircuit ifdesired. The circuit of Fig. 8 also differs from that of Fig. 1 in thatthe `frequency independent component of the automatic frequency control.potential is derived from the automatic volume control lead 21 insteadof the output resistor I4 of the detector I3.

'Ihe circuit of Fig. 9 differs from that of Fig. 1 in that the singlediodes I3 and`20 are replaced respectively by diode pairs 35i-4D and4|42 so as to secure voltage doubling as disclosed by Schade Patent No.2,072,278. A further differcontrol potential from a bridge circuitformed by the resistors I4 and 22 and a pair of resistors 43-44 to thecommon terminal of which the lead 23 is connected. The resistors 43 andMare not only chosen to attenuate alternating voltages from the lead 23but are also so proportioned that, at exact tuning, the potential dropsof resistors I4 and `22 are exactly balanced. As in the previousarrangements, mistuning results in a control potential which is appliedthrough the lead 23 to a suitable tuning control device `such as a localoscillator or the like.

I claim as my invention:

l. `An automatic frequency control circuit responsive to an amplitudemodulated carrier wave `including a transformer provided with primaryand secondary windings, a detector connected with said transformer toproduce a unidirectional potential component which is substantiallyindependent of the frequency of the impulses sup-n plied to saidcircuit, means connected with said transformer to produce aunidirectional potential component which varies with change in saidfrequency, and means providing a series connection between saidlast-named means and said detector for deriving the resultant of saidpotential components and the amplitude modulation output.

2. The combination of an automatic frequency control circuit including atransformer having input and output terminals, a detector connected tothe output terminals of said transformer,

. means for deriving from said detector a unidirectional potential whichis substantially independent of frequency within arelatively narrow passband, means for deriving from the input and output terminals of thetransformer a resultant unidirectional potential as the vector sum ofinput and output potentials whichis equal and opposed to said detectorunidirectional potential only at exact tuning, and means for combiningsaid unidirectional potentials to provide a controlling potential forsaid circuit. l

3. In a radio signal conveying system, the combination of a couplingtransformer having primary and secondary windings tuned to substantiallythe same signal frequency within a relatively narrow pass band offrequencies, a diode signal. rectifier device connected with theksecondary winding, means for applying modulated signals through saidtransformer to said diode rectifier, means for deriving from saidrectifier the detected modulation component of said signals, and aresultant direct current potential which isV substantially constant withchanges in `frequency and phase between said primary and selcondarywindings within `lsaid narrow pass band, a seconddiode rectifier devicecoupled to the primary and `secondary windings of said transformer toreceive the vector sum o-f the potentials in said primary and secondarywindings, means forwderiving from said second rectifier device a seconddirect current potential which varies with frequency and the phaserelation between the primary and secondary voltages, a control circuitconnecting said potential-deriving means for` said rectifiers in seriesrelation to each other, to derive `a resultant control potentialtherefrom. l

4. An automatic frequency control circuit including artransformerprovided with a primary and a secondary winding, a detector connectedwith said transformer to produce from one of said windingsaunidirectional potential component which is substantially independentof the frequency of the impulses supplied to said circuit within apredetermined relatively narrow pass band, means connected with saidprimary and secondary windings to produce `from both of said windings aunidirectional potential component which varies with change in saidfrequency within said band, and means for combining said potentialcomponents to provide a' re- Sultant controlling potential.

5. An automatic frequency control circuit including a transformerprovided with tuned pril mary and secondary windings, a detectorconnected with one of the windings of said transformer to produce auni-directional potential component which is substantially independentof the frequency of the impulses supplied to said circuit within arelatively narrow pass band of frequencies, rectifier means connectedwith both of said windings to produce a unidirectional potentialcomponent which varies withchange in said frequency from a meanfrequency within said pass band,` means connected with said detector andrectifier to combine said potential components to produce a thirdunidirectional potential component which changes in polarity in responseto departure of said frequency in different directions from exact tuningto said, mean frequency, and circuit means for deriving the resultant ofsaid potential components.

6. The combination of an automatic frequency control circuit including atransformer having input and output terminals, a detector connected tothe output terminals of said transformer, means for deriving from saiddetector a unidirectional potential which is substantially independentyof frequency within a relatively narrow pass band, means including amid-tapped tuning capacitor for deriving from said transformer aresultant unidirectional potential as the vector sum of input and outputpotential which is equal and opposed to said detector unidirectionalpotential only at exact tuning, and means for combining Vsaidunidirectional potentials.

7. lin a signal conveying system, the combination of a transformerhaving an input circuit and an output circuit, means for tuning saidinput and output circuits whereby at resonance `to a received signal thepotentials therein are substantially 90 out of phase with, one another,means for detecting signals in the output circuit of said transformer,additional means for detecting a resultant vector sum of signalpotentials in the input and output circuits of said transformer, andmeans for combining the deytectedl signal output of said detectingmeans.

8. In a signal conveying system, the combination of a transformer havingan input circuit and an output circuit, means for tuning said input andoutput circuits whereby at resonance to a received signal the potentialstherein are .sub-

stantially 90 out of phase with one another,

means for detecting signals lin the output circuit of `said transformer,additional means for detecting a resultant vector sum of signalpotentials in the input and output circuits of said trans` former, andmeans including a bridge network lcombining the between said first andsecond-named means for signal output of said detecting means. f 9. In asignal receiving system, the combination ofa transformer having an inputcircuit and an outputA circuit, means for'tuning said 'input and outputcircuits whereby at resonance to a received signal the potentialstherein are substantially`90 out of phase with one another, means fordetecting signals in the output circuit of said transformer, additionalmeans including a frequency doubler for detecting a resultant Vectorvsum of signal potentials in the input and outputy circuits ofsaid'transformer, and means for combining the signal output of saiddetecting means. f

10.` In a signal conveying system, the combination of a transformerhavingl anY input and an output circuit,means for tuning said input andOutput circuits whereby at resonance to a received signal the potentialstherein are substantially 90 out of phase with one another, means fordetecting signals in the output circuit of said transformer, additionalmeans including a fre,- quency 'doubler for detecting aresultant'vect'o-r sum of signal potentials inthe input and outputcircuits v-of said transformer, and means'A including ya bridge networkfor combining the signal output of said'detecting means; H Y

WINFIELD Ru KOCH 30

